can someone please explain how tissue fluid enters the capillary at the arterial end, and how it's returned at the venous end?

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Thank you

Posted Thu 15th November, 2012 @ 21:27 by SS

5 Answers

  • 2 votes

The hydrolytic pressure from the heart is greater at the arteriole end and is greater than water potential and so tissue fluid (which is blood plamsa minus the plasma proteins) leak out through the gaps in between the squamous epithelial cells that make up the capillary on to the tissue.

Then at the venule end, the pressure has dropped dramatically and so the water potential going into the capillary is now greater than the hydrolytic pressure (as the concentration of water is greater in the tissues than the capillary).

90% is drained back into the capillary but about 10% still remains in the tissues which gets drained into the lymphatic system and eventually re-enters the blood.

Answered Fri 16th November, 2012 @ 11:07 by Joanne
  • 2 votes

Water potential = osmosis. 

Osmosis is the movement of water from an area of high concentration to an area of low concentration of water.

The hydrolytic pressure is the pressure from when the heart beats blood into the artery (which is under high pressure). At the arteriole end, this force of pressure is greater compared to the 'water potential' so the plasma goes into the tissues which now becomes 'tissue fluid' (as it goes into the tissue).

Then at the venule end, this pressure is lower than the water potential (as a lot of pressure has been lost and the concentration of water in the tissues is high compared to the concentration of water in the capillary). This is why it drains back into the capillary at the venule end.

Hope it helps!

Answered Fri 16th November, 2012 @ 21:58 by Joanne
  • 1 vote

It is removed from the arteriolar end due to the pressure potential acting on the water potential. The pressure at which the blood plasma is carried via the arteriole end is higher than the on in the venous en, which creates a damming effect. This increases the water pot. allowing the blood the seep thru the gaps in the capillaries endothelium.
Almost 90% of the fluid seeped out eventually flows back in to the venous end due to the fact that the capillaries have a higher solute potential than the tissue cells. This is because most of the proteins can't fit thru the gap between the capillary in the first place.

Answered Fri 16th November, 2012 @ 09:16 by Didehan Altinisik
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Thank you, but I still don't get it :( Could you please tell me what solute potential is? Also could someone explain to me the basics. I really appreciate your help though Joanne and Didehan 

Answered Fri 16th November, 2012 @ 21:20 by SS
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When an artery reaches the tissues it branches into smaller arterioles, then into a network of capillaries, these eventually link up with venules to carry blood back to the veins. So blood flowing to an organ or tissue is contained in the capillaries. At the arterial end of a capillary, the blood is under high pressure due to the contraction of the heart muscle. This is known as hydrostatic pressure. It will tend to push the blood fluid out of the capillaries. The fluid can leave through the tiny gap in the capillary wall called fenestrations. The water can leave as the capillary endothelial wall is what we call 'squamous' endothelium (remember if you are talking about blood vessels they are ENDOTHELIAL cells not epithelial).

The fluid that leaves the blood consists of plasma with dissolved nutrients and oxygen. All the red blood cells, platelets and most of the white blood cells remain in the blood, as do the plasma proteins. These are too large to be pushed out through the gaps in the capillaries.

The fluid that leaves the capillary is known as the tissue fluid. This fluid surrounds the body cells, so exchange of gases and nutrients can occur across the cell surface membrane. This exchange occurs by diffusion and facilitated diffusion. Oxygen and nutrients enter the cells; carbon dioxide and other wastes leave the cells.

The hydrostatic pressure of the blood is not the only force acting on the fluid. The tissue fluid itself has some hydrostatic pressure which will push the fluid back into the capillaries.

Both the blood and tissue fluid contain solutes, giving them a negative water potential. Pure water has a water potential of 0 kPa. As other substances are dissolved in water this decreases the water potential and gives it a negative reading, the more negative a water potential the less water there is and more solutes. Water will always move towards the more negative potential.

The water potential of the tissue fluid is less negative than that of the blood. This means that water tends to move back into the blood from the tissue fluid via osmosis, down the water potential gradient. This fluid that moves back into the capillary carries any waste substances such are carbon dioxide that has left the cells. Despite what was said above this occurs at BOTH the arteriole and venuloe end of the capillary but the difference is that when your think of the NET flow of water it accounts for a larger proportion of movement at the venuole end.

Answered Thu 20th December, 2012 @ 18:52 by Rachel